Amongst the many types of compounds present in hops that are useful to brewers is a class of resinous compounds known as the α-acids. These compounds are primarily responsible for the bitterness of beer, being converted during wort boiling to their isomerized forms, known as iso-α-acids (Formula 1). The iso-α-acids are bitter and contribute also to the foam quality of the beer. Unfortunately, the conversion of α-acids to iso-α-acids in the wort kettle is rather inefficient and there are also subsequent losses of the iso-α-acids during the fermentation of the wort. Consequently, many brewers use preparations of iso-α-acids in forms that can readily be added to already fermented wort, thereby greatly increasing the utilisation of the original α-acids. These preparations can be efficiently manufactured from hop extracts, particularly from extracts prepared using liquid or supercritical carbon dioxide (CO2), and provide an easy and cheap way for the brewer to control the bitterness of beer. Although other preparations have been described, the normal form in which these iso-α-acids are made commercially available is as an aqueous solution of the potassium salts, most commonly as a slightly alkaline solution containing 30% of actual iso-α-acids by weight (i.e. 300 g per kg) or weight/volume (i.e. 300 g per liter) of the iso-α-acids.
Subsequent to the introduction of iso-α-acids solutions for post-fermentation bittering, a range of products was developed in which the iso-α-acids were converted to different forms of chemically reduced derivatives. These derivatives are also bitter, but the degree of bitterness varies from that of the iso-α-acids. Differences are also apparent in their ability to promote and modify the characteristics of beer foam. The commercially available, reduced forms of iso-α-acids also have the further property of resisting the light-induced breakdown of the iso-α-acid molecule that is a key factor in the development of “lightstruck” or “skunky” flavors in beers exposed to sunlight or some forms of artificial light. Hence, these chemically reduced compounds are also often used as the sole bittering agents in the preparation of beers that are sold in clear glass bottles.

Three types of reduced iso-α-acids are commercially available. These are the ρ-iso-α-acids (alternatively written as rho-iso-α-acids, Formula 2), the tetrahydroiso-α-acids (Formula 3) and the hexahydroiso-α-acids (Formula 4). (See European Brewery Convention Manual of Good Practice: Hops and Hop Products (1997), publ. by Getränke-Fachverlag Hans Carl, Nürnberg). Many different methods for the preparation of these compounds have been described, but a common feature of the manufacture of ρ-iso-α-acids is that they are prepared via reduction of iso-α-acids with an alkali metal borohydride, normally sodium borohydride. Tetrahydroiso-α-acids, on the other hand, are invariably made via catalytic reduction using a precious metal catalyst, commonly palladium on carbon, and hydrogen gas. Several different processes for the production of tetrahydroiso-α-acids have been described, and the starting material may be α-acids, iso-α-acids or even β-acids (substances that form an analogous sequence of compounds to the α-acids, but contribute little to the beer in normal brewing). Hexahydroiso-α-acids are prepared either by catalytic hydrogenation of ρ-iso-α-acids, or else by chemical reduction of tetrahydroiso-α-acids using an alkali metal borohydride. Tetrahydroiso-α-acids are substantially more bitter than are iso-α-acids; hexahydroiso-α-acids are also more bitter, but rather less so, while ρ-iso-α-acids are actually less bitter. In terms of foam enhancement, when compared on an equivalent bittering basis hexahydroiso-α-acids are generally considered the most effective, followed by the tetrahydroiso-α-acids. The iso-α-acids and ρ-iso-α-acids have similar, but substantially less foam enhancing effects when compared in this way. Worldwide, the particular combination of bittering and foam stabilising properties shown by the tetrahydroiso-α-acids has made this form of reduced iso-α-acid especially popular, either as a partial replacement for normal iso-α-acids in the production of beers having improved foam characteristics or for the brewing of light-stable beers.
Because they are sufficiently soluble in wort, ρ-iso-α-acids are often used via direct addition to the kettle as well as by addition to beer. Commonly, like iso-α-acids, they are available commercially as an aqueous, slightly alkaline solution at about 30% strength. Such solution is normally used, as are corresponding solutions of iso-α-acids. by dilution into demineralized water and subsequent injection into beer, though it is possible to make direct injections of the product itself if steps are taken to ensure sufficiently vigorous and rapid mixing. A concentrated form of this product is also available, in which the ρ-iso-α-acids are also in the potassium salt form, but at about 60% concentration. Such a composition is described in our co-pending U.S. provisional patent application No. 60/215,408, filed Jun. 30, 2000. It is an object of this invention to enable such a concentrated and necessarily rather viscous preparation of ρ-iso-α-acids to be used not only as a kettle additive but also, through provision of suitable means, as a post-fermentation additive or even as an in-line or direct kettle wort additive, there being no equipment presently available that is designed to facilitate such use.
Tetrahydro- and, particularly, hexahydroiso-α-acids are inherently less soluble than are iso-α-acids and ρ-iso-α-acids. For this reason, tetrahydroiso-α-acids are commonly sold as a 10%, slightly alkaline aqueous solution of their potassium salts. Similarly, preparations of hexahydroiso-α-acids are also sold as relatively dilute solutions, or else a solubilizing agent such as propylene glycol must be added—a course of action that is -considered unacceptable by most brewers. In the case of the tetrahydroiso-α-acids, Ting, in U.S. Pat. No. 5,874,633, described an improved manufacturing process whereby an aqueous, alkaline solution having a concentration of up to 45% could be obtained. However, Ting also reported (in column 4, lines 1–7) that, though the effect was reversible, this single phase solution would soon separate into two phases of distinctly different composition at temperatures below 28° C. Thus it may be expected that in most instances Ting's solution would not be physically stable when stored at ambient temperatures, and certainly not at the lower temperatures of a brewery cellar or coldstore. Clearly, it is inconvenient and more costly to use dilute or non-homogeneous solutions, and for this reason John Paul Maye in U.S. Pat. Nos. 5,583,262 & 5,624,701 describes the preparation of dry salts of isomerized and reduced isomerized α-acids, which he claims to enable a reduction of-costs because of the greatly reduced weight of material requiring to be shipped to the customer. However, it is obvious that the provision of dry, crystalline or powdery substances containing less than 2% moisture (as is indicated by Maye) requires extra work to be done in the brewery, since the materials must first be weighed out and then dissolved into water before they can be used. Furthermore, Maye's process for the production of these salts requires one to start with an aqueous solution of the iso-α-acids or reduced iso-α-acids, the water being subsequently removed by any one of a number of different methods. It is one of the further advantages of our invention to provide a means whereby such isomerized substances can be converted into highly concentrated yet fluid and readily useable forms from their free acid state without the need first to prepare such relatively dilute solutions, such forms being conveniently used to bitter wort or beer in ways that we later describe. Many brewers prefer to add bittering compounds to their worts rather than to the subsequent beer, because in this way they obtain some useful protection from Gram positive spoilage organisms whose activity is inhibited in the presence of isomerized α-acids of all types. However, direct addition of either tetrahydro- or hexahydro-iso-α-acids to the brewery kettle is not recommended, though may be occasionally practised, since the poor solubility of these compounds results in excessive losses due to precipitation into the trub. Nevertheless, it is an additional benefit of our invention that the concentrated products whose manufacture we describe are also particularly suitable for addition to the kettle, should the brewer so desire.
It is common practice when making a light-stable beer to use both ρ-iso-α-acids and tetrahydroiso-α-acids or hexahydroiso-α-acids as the bittering agents, the normal reason being to avoid producing a beer of the correct bitterness but that has an excessively stable foam. In U.S. Pat. No. 5,200,227, Guzinski & Stegink describe the production of stable, single phase, aqueous solutions of mixtures of two or more different types of isomerized α-acids. By means of preparing such mixtures, Guzinski & Stegink demonstrated that the amounts of tetrahydro- or hexahydroiso-α-acids that could be held in solution could be increased above the individual solubility limits of these types of compounds when prepared as aqueous alkaline solutions by themselves. This phenomenon was ascribed to an unexpected cosolvent effect. However, these authors also showed that, above certain limits, such mixtures were not physically stable and would form two phases, stating (in column 6, lines 30–34) that “ . . . there is an upper limit of concentration, at which the cosolvent effect is inoperative. This limit, for practical purposes, is about 45% by volume, and preferably the preparations are between 25% and 40% by volume in total concentration of iso-alpha acids”. Surprisingly, we have discovered that we are in fact able to readily prepare mixtures of different types of isomerized α-acids at much higher concentrations that are nonetheless homogeneous, have fluidity and therefore ideally suited to the working of our invention.